Lithium/sulfur (Li/S) batteries have been under study in the past decades owing to their low cost, relative abundance of the constituent elements, and their non-toxicity, as well as the ability to exhibit a much higher energy density than conventional lithium ion batteries (2567 Wh/kg vs. 387 Wh/kg). Lithium/selenium (Li/Se) batteries have likewise attracted an increasing amount of attention in recent years due comparable volumetric capacity and higher electronic conductivity in relation to Li/S batteries.
It has been reported that the electrolytes play a significant role in the electrochemical performance of sulfur and selenium based cathodes. Among them, carbonate-based and ether-based electrolytes are two types of popular electrolytes for selenium-sulfur based cathodes. Selenium has been found to be well-adapted to carbonate-based electrolytes, while sulfur does not. The nucleophilic reaction between polysulfides and carbonate electrolytes would be a major concern.
It was reported that ether-based electrolytes could facilitate the redox reaction of sulfur-based cathodes and generally offer higher reversible capacity than that of carbonate-based electrolytes. It was found that Se is reduced to the polyselenides, Li2Sen(n≥4), Li2Se2, and Li2Se sequentially during the lithiation process, and Li2Se is oxidized to Se through Li2Sen(n≥4) during the de-lithiation process in the 1st cycle, which undergoes similar reaction process to the sulfur system. However, even with a very good encapsulation of selenium or selenium-sulfur in the various carbon host materials, most of the previously reported Se-based cathodes show a gradual capacity fading in DOL-DME based electrolytes.
Our previous studies have revealed that the lithiation/de-lithiation reversibility of the selenium gradually decreased in DOL-DME based electrolytes, leading to an aggravated formation of long-chain polyselenides during cycling and further capacity decay. Moreover, Ab initio calculations revealed that the binding energy of polyselenides (Li2Sen) with carbon host is in an order of Li2Se6>Li2Se4>Li2Se. While the binding of polysulfides with the host is in order of Li2S>Li2S4>Li2S6, leading to the distinctive electrochemical performance. Therefore, the development of a good combination of novel cathode materials and electrolytes is required.